,1869.] On the genus Onchidium. 89 



can be removed from it without producing a change in the colour of the 

 animal. The surface of the mantle is generally finely granulated, but in 

 all our species some larger tubercles are besides found, more or less 

 numerous, and irregularly distributed bn it. These larger tubercles 

 can be protruded or retracted at will. When the animal is in a healthy 

 state, they are generally very distinct, each of them bearing one 

 to four jet black dots, the functions of which in the economy of the 

 animal it is difficult to understand, but most likely the pigment which 

 they contain, when added to the mucus secreted by the entire body, 

 acts as a kind of defensive fluid against other animals. The mantle is 

 amply supplied with nerves issuing from the central ganglion, but to 

 the touch, the tubercles do not appear to be much more or less sensitive 

 than the rest of the body ; they are always retracted when they come in 

 contact with a solid object, but soon protruded again. Sickly animals 

 not only change colour, but the body often shrinks to less than 

 half the original size, and all the tubercles of the surface are smoothed 

 down, and assimilated to the mass of the mantle. The mesial portions 

 of the mantle are usually thin, but the sides are very consistent and 

 fleshy, the muscular tissue being solid, very tough in some of the species 

 (0. tigrinum), soft, almost pulpy, in others, (0. tenerum). The inter- 

 nal fleshy part of the mantle is pure white, but the external parts, to 

 a smaller or greater thickness, blackened, and filled with pigment cells, 

 producing the various colours of the animal. Near the edge of the 

 mantle, there are usually some larger cavities in the tissue, as shewn 

 in the section of the portion of the mantle (fig. 3, plate xiv), evidently 

 allowing for an easier motion of these extreme edges. 



The foot is composed of numerous transverse muscles and is always 

 shorter and narrower than the mantle ; this varies, however, in the 

 different species. In some the foot is only one-third, or one-fourth, of 

 the width of the mantle, in others almost four-fifths of the same, setting 

 aside, however, those variations which merely depend upon the position 

 of the body. When the animal is at rest, — in a sort of contracted 

 position, — the width of the foot is in proportion smaller, than when the 

 animal moves about, in which case the mantle stretches out longitu- 

 dinally, while the narrowness of the foot appears to be more limited by 

 the transverse muscles. 



No generic importance can, strictly speaking, be attached either to 



12 



